As is well known to all experts in the construction of electric power lines, the usual modern method of tensioning electric conductors, either singly or in bundles, on pylons is the "controlled tension" method, using "pilot cords" in the initial stage.
In the more conventional method, the pilot cord is first substantially tensioned at ground level and then raised and conveyed over the pulleys suspended from the successive pylons in the line and then tensioned.
The actual tensioning operation begins only after the pilot cord has been tensioned and is consequently suspended between two pylons along the line. The actual tensioning operation normally comprises the following stages:
The pilot cord is recovered from one end whereas the other end is coupled to a haulage cable which, together with the pilot cord, is maintained under constant tension and is therefore always raised from the ground. The haulage cable is thus moved forward and progressively takes the place of the pilot cord on the pulleys.
Next, the haulage cable is recovered from one end whereas the other end is coupled to the electric conductor or bundle of conductors which, together with the haulage cable, are as before kept under constant tension and raised from the ground. The conductors are thus moved forward and progressively take the place of the haulage cable on pulleys. Finally, the pulleys conveying the pilot cord, haulage cable and conductors are replaced by known suspension insulators, to which the conductors are finally and permanently attached.
This method encounters considerable difficulties which may readily be understood and are well-known to those skilled in the art. They relate to the initial tensioning of the pilot cord and to the stretching of the haulage cables and conductors under constant tension.
The invention is mainly concerned with the problems and difficulties of tensioning the pilot cord. Although the tensioning process is relatively easy in flat open ground, it becomes more difficult if the route of the line goes through crops, which will inevitably be damaged, or woods, in which case it will be necessary to cut down entire lines of trees, particularly those with high trunks. The difficulties will be correspondingly greater if the route is still more irregular, as is frequently the case in mountainous regions or where the line crosses other structures such as streets, rivers, power lines, built-up areas or the like.
In an attempt to obviate these difficulties and the associated long periods of work, it has been proposed to use helicopters to tension the pilot cords. A helicopter moves along the route of the line above the pylons and gradually unwinds the pilot cord, which is conveyed by operators on the pylons over pulleys suspended therefrom or automatically runs on specially-shaped pulleys.
After the pilot cords have been stretched by the aforementioned method, the haulage cables followed by the conductors and/or bundles of conductors are stretched under constant tension in the aforementioned manner.
This method of stretching the pilot cords has the great advantage of reducing damage to vegetation along the route and thus shortening the time required, but is not free from disadvantages.
More particularly, the helicopter pilots have to be particularly skillful, firstly because the helicopter motion is opposed by the pilot cord, which is at least partly under tension; secondly because the pilot cord must be positioned on the pylons with high accuracy so that the operators can secure it or so that it can automatically run on the special pulleys; thirdly, owing to the need for accuracy, the helicopter has to fly at a very low altitude over the pylons or even remain still, with evident difficulty and danger; and last but not least, since the most important electric lines normally have six conductors or six bundles of conductors suspended from six brackets disposed at three different levels separated by several metres from one another, the difficulty and risk are multiplied by a factor of six (since one pilot cord has to be positioned for each conductor or bundle of conductors and one for the guard cable), or sometimes by an even higher factor if the helicopter, in order to accompany the pilot cords nearly as far as the lowest brackets, has to descend to an altitude at which the rotor blades are dangerously near the tip of the pylon or actually below it.
Of course, there is also the difficulty of manipulating large quantities of pilot cords and haulage cables. The helicopter and crew have to work for a long time, resulting in high depreciation and investment costs. For these reasons the aforementioned method and expenses are restricted to the most important lines for particularly difficult routes.